DESCRIPTION: The proposal seeks an understanding of the smallest biological motor known, the bacterial flagellar motor, which is widespread in both the eubacteria and the archaea. The motor has several distinctive features: (i) it is driven by the trans-membrane proton potential, not ATP; (ii) it undergoes true rotation; (iii) it can rotate in both the counterclockwise and clockwise directions, i.e., it has a switch; and (iv) the switch is controlled by the sensory transduction system to achieve chemotaxis. Thus the system permits exploration of several basic biological issues, such as sensory reception and transduction, controlled molecular switching, and electro-mechanical and osmo-mechanical energy transduction. The proposed research, to be carried out in Salmonella typhimurium, is basic in its nature. However, the information it has provided and will provide has been used by other researches working in more applied areas, which the role of motility and taxis in the virulence of microbial pathogens for humans (and other hosts including plants) and in environmental ecology (both on land and in the oceans) are under active investigation. The proposal makes heavy use of mutations (spontaneous and engineered) and or intergenic suppression, in the five proteins that comprise the motor. These are MotA, MotB, FliG, FliM, and FliN. It will build on existing knowledge concerning these proteins, with a view to identifying precisely the most critical regions and residues for (i) proton delivery to the site of torque generation (MotA and MotB); (ii) torque generation itself (the MotA-FliG interface); reception of sensory information by the switch (the CheY-FliM interface); and (iv) use of this information to switch motor direction (the FliM-FliG interface).